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Possible Host Key Failure Due to Key Size in Some Versions of SecureCRT 9.2.x and Newer

What causes the "server's host key failed to verify" error in some versions of SecureCRT 9.2.x and newer?

On the Windows platform, some versions of SecureCRT 9.2.x and newer are unable to verify SSH2 server host keys that have bit sizes which are not a multiple of 256 (e.g., Dropbear, some Cisco switches, some Cisco ASR routers, etc.). Connection attempts to such SSH2 servers with unusual host key bit sizes can result in the following error occurring during initial key exchange:

The client has disconnected from the server. Reason: The server's host key failed to verify.

In the case where this error is associated with an SSH2 server's host key having a bit size that is not a multiple of 256, the problem is most likely associated with a known issue caused by an updated cryptographic library used by SecureCRT version 9.2 and newer. See the "Known Issue" section of the following advisory:

RSA BSAFE Crypto-C Micro Edition vulnerabilities (CVE-2019-3728 and CVE-2019-3733) and VanDyke Client Products for Windows

While a newer encryption library is available from the vendor which resolves this issue, the newer library has not yet been certified with binary compliance to FIPS 140-2. FIPS certification is a process which takes an indeterminate number of months or years, so we are unable to provide an ETA for a fix that provides FIPS capability in SecureCRT.

Possible Solutions

Possible solutions involve one of the following options:

1. Replace the SSH2 server's host key with a new key which has a bit size that is a multiple of 256. If FIPS mode in SecureCRT is required and you want to be using SecureCRT version 9.2.x and newer, this option may be the only one that will work for you until a newer version of the BSafe Crypto-C ME library receives FIPS certification.


2. Install an alternate build of SecureCRT that includes a cryptography library which can verify signatures of host keys that have a bit size that is not a multiple of 256.
   Note: This option does not provide FIPS capability.


3. Downgrade to SecureCRT version 9.1.x or earlier. Prior versions of SecureCRT can be found in the Previous Releases section of the VanDyke Software website.

Note to SecureFX users: This documentation is written specific to SecureCRT, but also may apply to SecureFX versions 9.2 and newer for the SFTP and SCP protocols.